[v8volvo]

Welcome!

Looks like very nice work you did on the engine.

Uneven engine speed is often caused by air getting into the fuel system. Are there any places where air could be getting sucked in, for example near the fuel filter? Is the fuel filter installed tight and does not have a doubled up inner O-ring causing it to not seat the outer O-ring? Can you see any air bubbles moving in the translucent hose from the fuel filter mount up to the injection pump?

Do you smell diesel fuel under the hood anywhere? Any difficulty starting the engine if the car has been parked facing uphill? Those are also common signs of air leaking into the system.

Does the uneven operation happen all the time or just when the engine is cold?

If you are sure there is no air getting in, then the next place to look would probably be to double check injection timing and some other possibilities.